Radioactive and hazardous waste disposal continues to present technical, environmental and economic problems. Containment of such contaminated waste materials, which are often in the form of liquid solutions, wet slurries and particulate solids, such as soil particles or ion exchange resin beads, is difficult. In addition, the volume of contaminated wastes requiring disposal has been growing, while available storage space is limited. The cost of storing such wastes is therefore rising. Reducing the volume of the wastes is important to minimize costs. Major efforts have therefore been expended in recent decades to develop compact waste form materials with high resistance to cracking, pulverization, corrosion and leaching. Such efforts have met with limited success.
Borosilicate glasses, for example, have been used to contain wastes for disposal. Such glasses, however, have high melting temperatures (1100-1450.degree. C.). Many radioactive contaminants, such as cesium and ruthenium, have oxides which volatize at those high temperatures, requiring large off-gas treatment systems. In addition, the need for materials resistant to high temperature corrosion in the processing system and the high energy requirements of the system result in high costs.
Cement is also used to contain wastes. While containment in cement is a relatively inexpensive, low temperature process, cement is susceptible to leaching and cracking upon exposure to aqueous environments. In addition, cement enables only limited reduction in the volume of the contaminated material.
Incineration, vitrification, metal melt and pyrolysis have also been proposed for the immobilization and volume reduction of hazardous wastes. Incineration and vitrification, however, pose a significant risk of gaseous release of hazardous wastes to the environment. Currently, metal melt and pyrolysis are only available for the disposal of slightly contaminated wastes.
Resistance to corrosion is a basic consideration in the selection of appropriate materials for waste immobilization, as well as in the protective coating of materials. The susceptibility of various metals to corrosion is determined by their standard electrochemical potentials and the properties of their oxides. Aluminum, for example, has a high oxidation potential, but its oxide forms a continuous film which strongly adheres to the metal surface, protecting the aluminum metal against further attack. In the case of iron, oxides associated with both the +2 (ferrous) and +3 (ferric) oxidation states can be formed in corrosion processes. In pure water environments, the major corrosion product is the black ferrous-ferric oxide magnetite (Fe.sub.3 O.sub.4). Magnetite films on steel are moderately protective. In the presence of dissolved oxygen as well as chloride or sulfate ions, however, the corrosion products become fully oxidized to the ferric state. These corrosion products include a variety of more hydrated and less hydrated ferric oxides, which form non-adherent, flake-like, non-protective films. Thus, when iron and steel become covered with rust, the corrosion process is not arrested, but continues to attack progressively deeper regions.
The ultimate product of drying ferric oxides is hematite (Fe.sub.2 O.sub.3), which is a widely used pigment. The formation of hematite from hydrated iron oxides generally results in the formation of a loose, non-consolidated, powdered product.
Iron oxides have been used to immobilize hazardous materials. The resulting products, however, are powders. U.S. Pat. No. 5,221,323 to Li et al., for example, discloses methods of waste treatment by producing magnetic powders from heavy metal sludges by adding ferric compounds, such as ferric hydroxide or ferric oxide. The mixture is heated in an oven at 500-1400.degree. C. and then cooled. Li's examples are at 1,000-1,200.degree. C. The resulting powders are ground. A weak acid solution or inorganic sodium salts are added to the ground powders to aid in the separation of magnetic materials with a magnetic separator. The magnetic powders are then dried. Processes at temperatures above 500.degree. C., and particularly those over 1,000.degree. C., release volatile oxides which can contaminate the environment. Furthermore, powders stored in a containment vessel can be released into the air or the ground water if the vessel is damaged or disintegrates.
The precipitation of hydrated ferric oxide from aqueous solutions can be used to remove dissolved or suspended contaminants from such solutions through co-precipitation, sorption or both. A substantial fraction of certain contaminants, such as cobalt-60, can be removed from solution in this manner. The resulting material is typically a wet sludge or dried powder.
Powders have been treated to resist leaching by water. U.S. Pat. No. 4,601,832 to Hooykaas, for example, discloses treating waste material, including hazardous metals, with an acid solution of iron and preferably manganese to dissolve the hazardous metal. The waste material can be sludge, for example. The solution is made alkaline by the addition of ammonia to precipitate the hydroxides of the hazardous metals and the hydroxide of the iron or manganese. The mixture of hydroxides is air dried. Consolidation of the dried precipitate is specifically avoided. The particles are mixed with a water repellent substance, such as a polymeric silicon compound.
Contaminated materials containing iron compounds have been consolidated with binding materials. U.S. Pat. No. 4,508,641 to Hanulik, for example, discloses disposing of waste generated upon the decontamination of steel surfaces in radioactive facilities by treating the wastes, which contain iron, to precipitate insoluble iron compounds or iron hydroxide. The iron compounds are decomposed into iron oxide, which is consolidated in cement. U.S. Pat. No. 4,118,243 to Sandesara uses iron oxides to immobilize arsenic in solid or liquid waste material. The product is incorporated in a calcium sulfate matrix. The use of porous binding materials, such as cement or calcium sulfate, limits the amount of volume reduction which can be achieved. In addition, the presence of other materials may interfere with the formation of the matrix. Cement and calcium sulfate are also prone to leaching.
Protective ferric oxide coatings have been formed on surfaces by the treatment of the surface with acids. U.S. Pat. No. 2,728,696 to Singer, for example, discloses producing an adherent coating of hydrated ferric oxide on iron, steel and objects having a ferrous surface, by first forming a film of a dilute aqueous solution of acid or acid-reacting salt on the surface of the object to be coated. The film reacts with the surface of the object, yielding ferrous oxide, which is oxidized in humid air. U.S. Pat. No. 4,369,073 to Fukutsuka et al., discloses formation of a thin corrosion protective coating on the inner surface of a condenser tube made of a copper alloy. A thin layer of an acidic suspension containing iron powder is applied to the surface, which is then exposed to an oxidizing gas. The resulting film comprises ferric oxyhydroxides. The acid residues in the methods of Singer and Fukutsuka could cause local corrosion beneath the coating. Singer's method is also not applicable to coating materials which do not comprise ferrous alloys, such as stainless steel. Neither reference discloses the direct deposition of ferric oxide on a surface.
It is expected that the costs of immobilization and storage of hazardous wastes will continue to rise. A safe and secure immobilization process which is easy to implement and yields a solid product of reduced volume, is needed.